1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that perform error diffusion as pseudo-halftoning process on multivalued image data, and a program for implementing the image processing method.
2. Description of the Related Art
Conventionally, an error diffusing process has been known as a pseudo-halftoning process in which multivalued images are expressed as binary data (“An Adaptive Algorithm for Spatial Gray Scale” in society for Information Display 1975 Symposium Digest of Technical Papers, 1975, 36). In this process, where a notice pixel is designated by P, the density thereof is designated by v, the densities of peripheral pixels P0, P1, P2, and P3 around the notice pixel P are designated by v0, v1, v2, and v3, respectively, and a binarization threshold is designated by T, a binarization error E of the notice pixel P is distributed to the peripheral pixels P0, P1, P2, and P3 by respective weighting factors W0, W1, W2, and W3 obtained empirically so that the average density can be macroscopically equal to the density of the original image.
For example, where binary data to be outputted is designated by o, the output binary data, binarization error E, the respective densities v0, v1, v2, and v3 of the peripheral pixels P1, P1, P2, and P3 are expressed by:if v≧T, o=1 and E=v-Vmaxif v<T, o=0 and E=v-Vmin   (1)
(Vmax: maximum density, Vmin: minimum density)v0=v0+E×W0   (2)v1=v1+E×W1   (3)v2=v2+E×W2   (4)v3=v3+E×W3   (5)
(Examples of the weighting factors: W0= 7/16, W1= 1/16, W2= 5/16, and W3= 3/16)
To output a multivalued image using inks of four colors consisting of cyan (C), magenta (M), yellow (Y), and black (K), a color ink-jet printer or the like has conventionally carried out a pseudo-halftoning process in which error diffusion or the like is performed for individual colors independently of each other. Thus, even if visual characteristics are excellent in terms of a single color, favorable visual characteristics cannot be necessarily achieved in the case where two or more colors are overlapped.
To address this problem, Japanese Laid-Open Patent Publication (Kokai) No. 8-279920 and Japanese Laid-Open Patent Publication (Kokai) No. 11-10918, for example, have disclosed a pseudo-halftoning process in which error diffusion is performed on two or more colors to achieve favorable visual characteristics even if two or more colors are overlapped.
Japanese Laid-Open Patent Publication (Kokai) No. 09-139841 has also disclosed a process in which pseudo-halftoning is performed individually for two or more colors, and output values based on the sum of input values are then corrected to obtain the same effects as described above.
Particularly, to reduce the graininess of intermediate density regions of a color image, it is effective to form an image such that dots of the cyan component (C) and dots of the magenta component (M) are not overlapped. To this end, the following method has been employed.
FIG. 7 is a view useful in explaining a conventional color image processing method.
In FIG. 7, the image data of a color image is expressed as multivalued data in which coloring density components (YMCK) of each pixel are each composed of 8 bits (gradation values of 0 to 255). Where the density values of the C component and the M component of a notice pixel in a multivalued color image are designated by Ct and Mt, respectively, and the density values of the C component and the M component of the original image are designated by C and M, respectively, the density values Ct and Mt are expressed by:Ct=C+CerrMt=M+Merr
Here, Cerr and Merr represent accumulated error values obtained by error diffusion of the C component and M component, respectively, of the notice pixel.
According to the conventional color image processing method, control is provided in four ways according to the density values of the C component and the M component of a notice pixel as described below.
1. If the sum of Ct and Mt is equal to or less than a threshold (Threshold 1), i.e. if the sum of Ct and Mt lies inside a region 1 of FIG. 7, dot-recording using C (cyan) ink and M (magenta) ink is not performed.
2. If the sum of Ct and Mt is greater than the threshold (Threshold 1) and less than another threshold (Threshold 2) and Ct is greater than Mt, i.e. if the sum of Ct and Mt lies inside a region 2 of FIG. 7, dot-recording using only C ink is performed.
3. If the sum of Ct and Mt is greater than the threshold (Threshold 1) and less than the other threshold (Threshold 2) and Ct is equal to or less than Mt, i.e. if the sum of Ct and Mt lies inside a region 3 of FIG. 7, dot-recording using only M ink is performed.
4. If the sum of Ct and Mt is equal to or greater than the other threshold (Threshold 2), i.e. if the sum of Ct and Mt lies inside a region 4 of FIG. 7, dot-recording using C ink and M ink is performed.
It should be noted that the relationship between the above-mentioned two thresholds is expressed by Threshold 1<Threshold 2.
However, the above described conventional image processing method is intended to reduce the overlap of ink dots of different colors such as cyan and magenta, but cannot eliminate factors that can deteriorate other visual characteristics as described below.
1. The overlap of dark ink dots and light ink dots of the same color system with different coloring densities.
2. The overlap of large droplet ink dots and small droplet ink dots of the same color system with different discharge rates.
3. The overlap of dots of the entire ink system including an ink group consisting of the inks in the above 1. and 2. and a plurality of color inks in the prior art.
Particularly, the overlap of the inks in the above 1. and 2. deteriorates visual characteristics due to the overlap of dots, generates a pseudo outline in half-tone, and increases the quantity of inks shot for reduction of the pseudo outline, thus raising the running cost.